The present invention relates generally to a distance measuring apparatus used in autofocus systems for cameras, human body detectors in crime prevention systems and the like.
More particularly, the invention relates to an active type distance measuring apparatus for measuring a distance by emitting an infrared beam from a light emitting portion and receiving the infrared beam reflected by a reflective object by means of a light receiving portion.
One known active type distance measuring apparatus for judging whether a reflective object is located at a far point or at a near point relative to a certain preset distance is disclosed, for example, in Japanese Unexamined Patent Application No. 59-148033. This apparatus measures a distance from a far point when the output signals of two phototransistors decrease, so that the far point is used for distance measurement when the output signal is insufficient.
A conventional active type distance measuring apparatus requires an amplifier having high amplification and a comparator, resulting in the drawback that the circuit structure becomes complicated and adjustment is difficult. In particular, such a simple distance measuring apparatus which differentiates between far and near points has had a problem in that the cost thereof is considerably high.